1. Field of the Invention
The present invention relates to a maintenance liquid for inkjet printers and a method for cleaning inkjet print heads and inkjet printers using the same.
2. Description of the Related Art
Printing systems, such as offset and flexography, are being increasingly replaced for printing applications, such as billboards, packaging and decoration, by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing, and due to their enhanced reliability, allowing their incorporation into production lines. Radiation curable inkjet inks are particularly preferred because high quality images can be printed on non-absorbing ink-receivers.
In an industrial environment, the inkjet inks in the inkjet printer can be replaced with a different type of inkjet inks for having e.g. a superior adhesion result on a difficult substrate, or an inkjet printer can be set on non-active because of too high production capacity. In such cases of an ink swap or a long standstill, the inkjet ink is normally discharged from the inkjet printer and the inkjet printer is cleaned and filled with maintenance liquid.
Such maintenance liquids are well-known in the art for aqueous inkjet ink printers. For example, US 2007132807 A (SEIKO EPSON) discloses a maintenance liquid for ink jet recording comprising water, a resin solvent, and a moisturizing agent.
EP 1621348 A (TOSHIBA TEC) discloses a washing solution for washing an UV curable inkjet ink inkjet printer head, which contains not less than 50 parts by weight of a polymerizable compound selected from the at least two kinds of polymerizable compounds included in the ink and having the lowest viscosity among the at least two kinds of polymerizable compounds, or not less than 50 parts by weight of a polymerizable compound having a viscosity of 30 mPa·s or less at ordinary temperature. The use of polymerizable compounds in the washing solution is not only an expensive solution, but also holds the risk of undesired polymerization of the washing solution in the print head and the ink circuit.
EP 1837182 A (FUJIFILM) discloses an ink washing liquid for a photocurable ink comprising at least one type of ether compound for cleaning the surroundings of the discharge orifice of the inkjet print head (see [0006]). For this reason, the ether compound preferably has a boiling point of 50° C. to 150° C. (see [0030]) so that little residual washing liquid remains on the nozzle plate of the inkjet print head after cleaning. Due to their fast evaporation, such washing liquids are not really suitable as a maintenance liquid for filling the inkjet print head and the ink circuit of an inkjet printer for a prolonged time.
EP 2157163 A (TOYO INK) discloses a maintenance liquid for inkjet printers comprising at least one of glycol ethers and glycol esters and 45 to 10 mg/L of dissolved oxygen. A large amount of oxygen dissolved in the maintenance liquid may lead to the creation of gas bubbles in the ink circuit of the inkjet printer. Nowadays, industrial inkjet printers include for reliable inkjet printing a degassing unit as, for example, shown in FIG. 7 WO 2006/064036 A (AGFA). The degassing unit is normally also used in the cleaning operation of the inkjet printer for preventing gas accumulation in some parts of the ink circuit, where subsequently ink may dry or solidify.
There is a still a need for an improved maintenance liquid which has high stability, excellent cleaning properties and is suitable for filling the print head and ink circuit during a long standstill of the inkjet printer.